Add FromIterator and IntoIterator impls for ThinVec

These should make using `ThinVec` feel much more like using `Vec`.
They will allow users of `Vec` to switch to `ThinVec` while continuing
to use `collect()`, `for` loops, and other parts of the iterator API.

I don't know if there were use cases before for using the iterator API
with `ThinVec`, but I would like to start using `ThinVec` in rustdoc,
and having it conform to the iterator API would make the transition
*a lot* easier.

I added a `FromIterator` impl, an `IntoIterator` impl that yields owned
elements, and `IntoIterator` impls that yield immutable or mutable
references to elements. I also added some unit tests for `ThinVec`.

compiler/rustc_data_structures/src/thin_vec/tests.rs

@@ -0,0 +1,42 @@
+use super::*;
+
+impl<T> ThinVec<T> {
+    fn into_vec(self) -> Vec<T> {
+        self.into()
+    }
+}
+
+#[test]
+fn test_from_iterator() {
+    assert_eq!(std::iter::empty().collect::<ThinVec<String>>().into_vec(), Vec::<String>::new());
+    assert_eq!(std::iter::once(42).collect::<ThinVec<_>>().into_vec(), vec![42]);
+    assert_eq!(vec![1, 2].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2]);
+    assert_eq!(vec![1, 2, 3].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2, 3]);
+}
+
+#[test]
+fn test_into_iterator_owned() {
+    assert_eq!(ThinVec::new().into_iter().collect::<Vec<String>>(), Vec::<String>::new());
+    assert_eq!(ThinVec::from(vec![1]).into_iter().collect::<Vec<_>>(), vec![1]);
+    assert_eq!(ThinVec::from(vec![1, 2]).into_iter().collect::<Vec<_>>(), vec![1, 2]);
+    assert_eq!(ThinVec::from(vec![1, 2, 3]).into_iter().collect::<Vec<_>>(), vec![1, 2, 3]);
+}
+
+#[test]
+fn test_into_iterator_ref() {
+    assert_eq!(ThinVec::new().iter().collect::<Vec<&String>>(), Vec::<&String>::new());
+    assert_eq!(ThinVec::from(vec![1]).iter().collect::<Vec<_>>(), vec![&1]);
+    assert_eq!(ThinVec::from(vec![1, 2]).iter().collect::<Vec<_>>(), vec![&1, &2]);
+    assert_eq!(ThinVec::from(vec![1, 2, 3]).iter().collect::<Vec<_>>(), vec![&1, &2, &3]);
+}
+
+#[test]
+fn test_into_iterator_ref_mut() {
+    assert_eq!(ThinVec::new().iter_mut().collect::<Vec<&mut String>>(), Vec::<&mut String>::new());
+    assert_eq!(ThinVec::from(vec![1]).iter_mut().collect::<Vec<_>>(), vec![&mut 1]);
+    assert_eq!(ThinVec::from(vec![1, 2]).iter_mut().collect::<Vec<_>>(), vec![&mut 1, &mut 2]);
+    assert_eq!(
+        ThinVec::from(vec![1, 2, 3]).iter_mut().collect::<Vec<_>>(),
+        vec![&mut 1, &mut 2, &mut 3],
+    );
+}